1. Field of the Invention
The invention relates to an apparatus and method for determining the vapor pressure of heavy hydrocarbon stocks such as heavy crude oil and residual fractions.
2. Description of the Prior Art
Vapor pressure specifications are used in oil processing for quality control purposes and for monitoring compliance with safety and regulatory regulations. For instance, air pollution control regulations specify maximum permissible vapor pressures in oil storage tanks. Accordingly, there is a continuing need to determine the vapor pressures of hydrocarbon stocks such as crude oils from petroleum, shale, coal, tar sands and other sources and fractions of such oils.
Many vapor pressure tests are suitable for determining the vapor pressure of low boiling stocks such as gasoline and light distillates. When used on heavy stocks, however, these tests typically give erroneous measurements due to the presence of dissolved air or traces of moisture or the inability to accurately determine the contribution of small amounts of light components. For instance, the Reid vapor pressure test (ASTM D-323-79) is not sensitive enough for use with heavy crude oils and residual stocks. When used on such heavy stocks a substantial portion of the observed pressure is attributable to dissolved air and traces of moisture. Other manometric techniques, such as the isoteniscope test (ASTM D-2879-75), also give erroneous readings with heavy stocks due to the presence of dissolved air.
Various dynamic methods for measuring vapor pressure are described in the Encyclopedia of Chemical Technology, Kirk-Othmer 2nd Ed., Vol. 21 pp 230-238 (1970). Most of the described methods involve vaporliquid equilibrium stills. Jentoft et al, Rapid Determination of the Vapor Pressure of Lubricating Oils and Hydraulic Fluids, Anal. Chem. 40, 1014 (1968) describes a dynamic vapor pressure technique that is designed to determine the bulk vapor pressure of a liquid and to ignore the effect of traces of light components. It is, however, not appropriate for determining the vapor pressure of heavy stocks because it ignores the contribution of light components.
It is also known that the vapor pressure of petroleum fractions can be determined using gas chromatographic analysis. In Eggertsen F. T., et al, Estimation of the Vapor Pressure of Petroleum Distillate Fractions from Gas Chromatographic Data, Anal. Chem. 52, 2069-2072 (1980), a sample of the fraction is analyzed chromatographically to develop a chromatogram. The chromatogram is compared to chromatograms of standard compounds of known carbon content and is divided into areas by carbon number. The area of each carbon number segment on the chromatogram is determined and the mol fractions of the components are calculated from the areas. Partial pressures are obtained by multiplying the mol fraction by the saturation pressure determined using the Antoine equation. The vapor pressure is calculated by summation of the partial pressures.
Application of the above described gas chromatograph technique to measuring the vapor pressure of a heavy crude or residual fraction poses two problems. Firstly, with such stocks elution from the chromatograph is incomplete, thus making it impossible to calculate the mol fractions of the lighter components from the chromatogram. Secondly, the analysis is based on the liquid phase. Therefore, to calculate the corresponding vapor pressure the fugacities for each component of the liquid sample must be known. They are typically not known and fugacities must be assumed on the basis that there are no azeotropes present between the components of the sample.
A principal object of the present invention is to provide an apparatus and method for determining the vapor pressure of heavy hydrocarbon stocks accurately with relative ease and simplicity.